Thus the conversation ended, but I think the point was clear enough. If an infinite series of events, causes, numbers, etc have to pass before any event, that event can never in principal come to pass. If we know a certain event did come to pass, we then also know only a finite number of event, causes, etc proceeded that event. Thus, if something, anything, is caused today we know it could in principal only have been proceeded by a finite number of causes. Any finite number of things begins with a first in the series. Thus there must be a first cause that is itself uncaused (lest it not be the first cause). This first cause is what we theists call God.
Therefore, we either have nothing ever being caused (which is absurd) and atheism being true or we have things being caused (which is self evidently true) and God.
Solamon Grundy, before breaking off the conversation, touched on a separate issue - the relation between God and time. He seemed to be suggesting that an eternal being, God, would have lived through an infinite past number of days. Such, of course, isn't what theists believe at all about the relationship between God and time. No one presents this quite as elegantly as Frank Sheed in Theology and Sanity,
What then is time?... Time... is the duration of that which changes; time... is the measurement of the changes of the universe.... Where nothing changes, there is nothing for time to measure. Where nothing changes, time has no possible meaning. Thus time and the universe started together. God is infinite and therefore changeless.... The universe He created is a changing universe. And because change belongs to it and not to God, time belongs to it and not to God.... time is the ticking of the universe.
Thus the phrase "before the universe was created" has no meaning at all. Before is a word of time, and there could be no time before the universe. To say "before the universe" means when there wasn't any "when"; which is to say that it doesn't mean anything at all. ...
Apply all this to the consideration of one further absurdity that tends to shadow the back of our minds, even when in the front of our minds we are by way of knowing better. It is the vague feeling we have that eternity had been going on for some time before God decided to create the universe. In light of what we have said, this is seen to be sheerly meaningless, for it brings time into eternity. We must not think of God creating the universe after a certain amount of eternity had rolled by, because there are no parts in eternity, and it does not roll by. This mental monstrosity is perhaps related to the picture of God as an old man. But God is not only not an old man, He is not even an old God. He is not old at all. For "old" simply means that one has lived through a long time; and there is no time to God. (pp. 66-68)God, then, hasn't lived through an infinite number of "past days" or for a limitless time. Rather He lives in one present, changeless "now," an eternal instant which has no beginning, middle, or end - that doesn't have any time to it at all, but that stands apart from and outside of all time (which has no meaning where nothing changes). God then isn't an exception to the impossibility of an infinite regress rule (which would itself be impossible), but stands in a qualitatively different relationship to time altogether (one which doesn't require the passing of time at all).
The whole article at Reduction into Modernism is worth checking out.